The atomic bomb destined for the ancient trading port of Nagasaki was called Fat Man. Sister Regina McKenna, my grandfather's sister, was close enough to ground zero to feel the death wind on her face. She might have preferred another name: Terror. Or, as the Japanese call it: Slaughter.

We knew her as Aunt Reggie, a nun with Montreal's order of the Sacred Heart. She was teaching in Japan when war broke out. She was imprisoned in an internment camp outside Nagasaki.

Boasting some beautiful churches, Nagasaki was home to Japan's biggest colony of Christians. Thus far, the city had been spared the U.S. Army Air Force campaign of annihilation. Under General Curtis LeMay, armadas of B-29s were systematically bombing Japan "back to the stone age."

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The bomb was a Rube Goldberg device consisting of two armoured ellipsoids, bound together with hoops of steel. The device was loaded in the forward bomb bay of a four-engine B-29. The device detonated over Hiroshima on Aug. 6 had been judged a stunning success. The single atomic blast killed at least 140,000 – mostly old men, women and children.

As a warning, the U.S. Army Air Force dropped millions of leaflets over Japan spelling out what the bomb did to Hiroshima: "The equivalent in explosive power to what 2,000 of our giant B-29s can carry on a single mission."

The B-29 carrying Fat Man took off from Tinian in the Pacific, at 3:47 a.m. on Aug. 9. Great flashes of lightning tore open the sky. St. Elmo's fire danced across the wing tips. The first target was the Kokura arsenal on the island of Kyushu. If the weather proved forbidding, the backup target was Nagasaki, Japan's answer to fashionable San Francisco.

In Japan's early trading days, Dutch and Portuguese traders came and went from the port. Ironically, the torpedoes that triggered the war, attacking the U.S. fleet at Pearl Harbor in 1941, were manufactured at the Mitsubishi works in Nagasaki.

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Now the city's fate was in the hands of a U.S navy commander, Fred Ashworth. It was he who would make the call on whether to drop on Kyushu, or switch to plan B, and annihilate Nagasaki.

Seventeen thousand feet below, on the hills of Nagasaki, my Aunt Reggie got permission to gather hay to feed the camp cow. She noted the camp guards had become jumpy, and a lot more kindly.

In her letter dated Sept. 12, 1945, and smuggled home, she wrote there was no warning.

"I think the approach of a solitary plane deceived the Japanese … I looked up to see if it were visible, but quickly decided that it would be wiser to hurry back to the camp … I began to run."

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Ashworth found that the first target on Kyushu was fogged in. The B-29 headed for Nagasaki, only to discover it too was socked in. But then a hole opened up in the clouds. Ashworth estimated he had a 20-second window to deliver Fat Man.

In those seconds, the fate of an ancient city was sealed.

Fat Man was released, flying like an angel of death toward an unsuspecting population. The bomb exploded at 500 metres at precisely 11:02 a.m.

"It seemed as though the sun had burst and I was lost in its midst. I threw myself at once into a clump of young bamboo trees. I was lying on a big bag of grass. My face only felt hot."

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She was spared the worst of the physical blast, shielded by the tall hills encircling Nagasaki. She ran back to the camp. All the windows had imploded. The shattering glass inflicted minor wounds. Downtown Nagasaki was another story.

The city burned for days. Almost all of the churches were destroyed. The survivors wrote of unspeakable suffering, 70,000 dead within 90 days, and everywhere a stench of death and corruption.

Sister McKenna wrote that the medical system is overwhelmed. "Two-thirds of the population of Nagasaki are dead. The city itself is a mass of ruins. They are still burning the dead. The hospitals having been destroyed, the wounded are not being cared for."

In 1945, the medical world knew little of radiation poisoning, so her next line resonates. "Some patients apparently recover, then suddenly die from hemorrhages."

All reporting from Nagasaki was censored.

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A Japanese and American film crew rushed to the site. But their footage was suppressed for more than 50 years. When footage finally aired, the impact on public policy ("why two bombs?") that the nuclear attack should have triggered was diminished by the passage of time.

For Sister McKenna, the imprint of that explosion was like a gigantic X-ray, searing her body and mind for a lifetime. Her fellow teaching sisters said "she was never quite right" after experiencing that cataclysm. And in the end she succumbed to a cancer brought on by radiation poisoning.

On a university field trip to Japan, my daughter Kate searched for her grave. She found it at the Sacred Heart convent at Susono, her memory kept green by a new generation of teaching sisters.

In the generations since Nagasaki, it's a miracle that we have somehow avoided nuclear war. It is estimated that a nuclear exchange would kill at least half of humanity, creating a nuclear winter, a world of death, where as the famous saying goes, "the living would come to envy the dead."

Brian McKenna lives in Montreal.

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Editor's Note: The original print version of this article and an earlier online version incorrectly attributed a quote to John Kennedy. The author of the quote is in dispute, but the observation that "the living would come to envy the dead" has been attributed by various sources to former Soviet Union leader Nikita Krushchev and military historian Herman Kahn and a Russian translation of Treasure Island by Nikolay Chukovsky. This online version has been corrected.